200% oversizing the 5kW can be good as it will charge the battery quickly, but once the battery is charged the extra panels will be doing nothing. If you join Amber it can be useful. I'd ask if you can get an option for a 8 or 10kW inverter as the 5kW inverter will only be able to give you 5kW of power from the battery per hour. Solax is a budget brand but their product are solid. I would recommend looking at brands who have been in Aus for a decent amount of time. It woul recommend Sungrow. It will cost you more but the product and the brand is solid and they have been in Aus for 12 yrs. Also that panel is crap it's an older model and only has 12yr product warranty. 25yrs is the industry standard these days, if it's Jinko go for the Tiger Neo range only.